There is known a bipolar transistor containing a group III nitride semiconductor as a main material. FIG. 1 is a sectional view showing a typical configuration of a bipolar transistor. Such a bipolar transistor is reported in, e.g., L. S. McCarthy et al., “AlGaN/GaN Heterojunction Bipolar Transistor”, IEEE Electron Device Letters, Vol. 20, No. 6, pp. 277, (1999).
In FIG. 1, the bipolar transistor includes a sapphire substrate 100, a sub-collector layer 103 including high density n-type GaN, a collector layer 104 including low density n-type GaN, a base layer 105 including p-type GaN and an emitter layer 106 including n-type Al0.1Ga0.9N. A crystal growing direction with respect to a substrate surface is in parallel to a [0001] direction. An emitter electrode 10E contacting the n-type AlGaN emitter layer 106, a base electrode 10B contacting the p-type GaN base layer 105 and a collector electrode 10C contacting the n-type GaN sub-collector 103 are respectively formed.
As a related technique, a p-type nitride semiconductor structure and a bipolar transistor are disclosed in the international patent publication No. WO2004/061971 (corresponding to U.S. patent publication: US2005/224831(A1)). This p-type nitride semiconductor structure is provided with a p-type nitride semiconductor layer including In that is re-grown on the p-type nitride semiconductor subjected to an etching process.
Further, a bipolar transistor and a manufacturing method thereof are disclosed in Japanese patent publication No. JP2006-128554(A). This bipolar transistor includes a first semiconductor layer, a second semiconductor layer and a third semiconductor layer. This bipolar transistor is provided with an insulating film having an opening portion formed under the second semiconductor layer and a part of the first semiconductor layer formed in contact under the insulating film.
Further, a nitride semiconductor bipolar transistor and a manufacturing method of a nitride semiconductor bipolar transistor are disclosed in Japanese patent publication No. JP2008-4779(A). This nitride semiconductor bipolar transistor is provided with a nitride semiconductor layer formed in contact with an emitter electrode or a collector electrode, including InAlGaN quaternary mixed crystal.
This time, the inventors have now discovered the following facts. FIG. 2 is a diagram showing an energy band of the bipolar transistor shown in FIG. 1. This diagram of the energy band shows the case where a forward bias is applied between the base and the emitter and a reverse bias is applied between the base and the collector. According to a band calculation, it is known that there exist an L-M valley and a second gamma (Γ) valley in high energy side by approximately 2.0 eV from a bottom of a conduction band of GaN, i.e., gamma (Γ) valley. These are collectively called “upper valley” depicted in FIG. 2 (represented by a dashed line in the figure).
As shown in FIG. 2, an electric field intensity becomes maximum in the vicinity of an interface between the base layer 105 and the collector layer 104 in the bipolar transistor shown in FIG. 1. Therefore, electrons injected to the collector layer 104 from the base layer 105 become high energy and undergo phonon scattering to be easily transited to the upper valley. Consequently, this bipolar transistor has a tendency that a carrier speed at a high voltage operation is lowered so as to lower a cutoff frequency. Moreover, since electrons are apt to have high energy within the collector layer 104 and GaN composing the collector layer 104 has a relatively narrow band gap, an avalanche breakdown easily occurs. Therefore, this bipolar transistor is involved with a problem that a collector breakdown voltage is low.